Interesting discussion on SELCAL. I am surprised the system provided alert on HF system when a VHF frequency was selected.Normally the ATIS is on VHF frequency.
The SELCAL decoder inside AMU should have provided alert on VHF if spurious tones randomly coincided with selcode of that aircraft.
In case of Boeing aircrafts SELCAL decoder is a separate box, it is not part of AMU.
You can get indications similar to SELCAL(call light on ACP and chime) when you receive a message from ground station on ACARS . ACARS uses Center VHF (third VHF on Boeing aircraft) .